1. Field of the Invention
The invention relates to a rotation angle detection device that detects a rotation angle of a rotary body, and an electric power steering system including the rotation angle detection device.
2. Description of the Related Art
As a rotation angle detection device that detects a rotation angle of a rotary body, there has been known a rotation angle detection device that detects a rotation angle of a rotor of a brushless motor with the use of a detection rotor that rotates in accordance with the rotation of the brushless motor. Specifically, as illustrated in FIG. 15, a detection rotor 201 (hereinafter, referred to as “rotor 201”) includes a cylindrical magnet 202 having a plurality of magnetic pole pairs corresponding to magnetic pole pairs formed in a rotor of a brushless motor. Two magnetic sensors 221, 222 are arranged around the rotor 201 at a predetermined angular interval around the rotation central axis of the rotor 201. The magnetic sensors 221, 222 respectively output sinusoidal signals having a predetermined phase difference. On the basis of the two sinusoidal signals, a rotation angle of the rotor 201 (a rotation angle of the brushless motor) is detected.
In this example, the magnet 202 has five magnetic pole pairs. That is, the magnet 202 has ten magnetic poles arranged at equal angular intervals. The magnetic poles are arranged at angular intervals of 36° (180° in electrical angle) around the rotation central axis of the rotor 201. Further, the two magnetic sensors 221, 222 are arranged at an angular interval of 18° (90° in electrical angle) around the rotation central axis of the rotor 201.
The direction indicated by an arrow in FIG. 15 is defined as the forward rotation direction of the detection rotor 201. The rotation angle of the rotor 201 increases as the rotor 201 is rotated in the forward direction, whereas the rotation angle of the rotor 201 decreases as the rotor 201 is rotated in the reverse direction. The magnetic sensors 221, 222 output sinusoidal signals S1, S2, respectively. As illustrated in FIG. 16, one period of each of the sinusoidal signals S1, S2 corresponds to a duration in which the rotor 201 rotates an angle of 72° (360° in electrical angle) corresponding to one magnetic pole pair.
The angular range corresponding to one rotation of the rotor 201 is divided into five sections corresponding to the five magnetic pole pairs, a start position of each section is defined as 0°, and an end position of each section is defined as 360°. A rotation angle of the rotor 201 expressed under the above-described conditions is an electrical angle θ of the rotor 201. In this case, the first magnetic sensor 221 outputs an output signal of S1=A1·sin θ, and the second magnetic sensor 222 outputs an output signal of S2=A2·cos θ.
Each of A1 and A2 represents an amplitude. If the amplitude A1 of the output signal S1 and the amplitude A2 of the output signal S2 are assumed to be equal to each other, the electrical angle θ of the rotor 201 is obtained with the use of both the output signals S1, S2 based on the following expression.
                    θ        =                ⁢                              tan                          -              1                                ⁡                      (                          sin              ⁢                                                          ⁢                              θ                /                cos                            ⁢                                                          ⁢              θ                        )                                                  =                ⁢                              tan                          -              1                                ⁡                      (                                          S                1                            /                              S                2                                      )                              
The thus obtained electrical angle θ is used to control the brushless motor. Refer to, for example, Japanese Patent Application Publication No. 2008-26297 (JP 2008-26297 A).
In the above-described conventional rotation angle detection device, the rotation angle θ is computed on the assumption that the amplitudes A1, A2 of the output signals S1, S2 output from the magnetic sensors 221, 222 are equal to each other. However, the amplitudes A1, A2 of the output signals S1, S2 vary depending on variations of the temperature characteristics of the magnetic sensors 221, 222 and temperature changes. Therefore, an error may be caused in detection of a rotation angle of the rotor due to variations of the temperature characteristics of the magnetic sensors 221, 222 and temperature changes.